gastric emptying

There is more to effective hydration than simply ingesting fluid, as anyone who has read Andrew Hamilton’s lead article in this issue should now be aware. Little benefit can be derived from an ingested drink until its components have been incorporated into the relevant body pool. And this, in turn, depends on the rate at which the ingested drink is emptied from the stomach, absorbed by the small intestine and transported onwards.

The rate of gastric emptying may be affected by a number of factors, including the composition of the drink and the activity being performed. And a new study from the UK and New Zealand was set up to investigate two key questions:

1. Whether gastric emptying was slowed during a standardised variable-intensity shuttle running test designed to simulate the activity pattern of football.
2. Whether low-energy fluids were emptied more effectively than carbohydrate solutions during this kind of intermittent exercise.

In a study on eight healthy males, the researchers compared the volume of test drink emptied during two 15-minute periods of walking exercise with the volume emptied during two 15minute periods of the Loughborough Intermittent Shuttle Test (LIST). For each type of exercise, the subjects completed one bout after ingesting a set amount of a carbohydrate-free placebo and the other after ingesting the identical amount of a 6.4% carbohydrate-electrolyte sports drink.

Gastric emptying was measured before and after each of the four trials using a technique that required them to swallow a piece of equipment known as a gastric aspiration tube.

The researchers found that gastric emptying was slowed during the LIST trials by comparison with the walking trials, although the overall exercise intensity would not normally be considered sufficient to affect gastric emptying had it been carried out at a constant power output. In fact, the subjects were exercising at or above 70% of their V02max for only about three minutes, 21 seconds during each 15-minute period of the LIST, and for the rest of the time at or below 50% V02max – similar to the intensities measured in professional footballers competing in outdoor matches.

‘This finding suggests,’ say the researchers, ‘that even a small amount of high-intensity sprinting can slow gastric emptying during variable-intensity exercise such as occurs during many sports.’

Most studies have found that solutions with a carbohydrate content of 6% or more empty more slowly than energy-free drinks when ingested at rest or during low-to-moderate intensity constant output exercise. And, indeed, in this study the volume of placebo drink emptied over 30 minutes during the walking trials was greater than that of the carbohydrate drink.

However, during the LIST trials both drinks emptied at the same slowed-down rate, which indicates, say the researchers, that exercise has a greater effect on gastric emptying than carbohydrate content at this intensity.

‘This present study,’ they point out, ‘is the first to conclusively show that variable-intensity exercise, which simulates the activity pattern of soccer, slows the rate of gastric emptying of energy-free drinks such as water to the same extent as that of carbohydrate-containing sports drinks.

They conclude that, given the potential benefit of ingested carbohydrate in delaying the fatigue process, and also in promoting intestinal water absorption, there seems little reason to advocate drinking water rather than a suitable sports drink during a football match.

Med Sci Sports Exerc, vol 37, no 2, pp240-247, 2005